feat: add case reorder and delete in Manage tab

Add a "Manage" tab to the create-case modal with up/down reorder
buttons and delete for each case. Linked cases are unlinked (folder
preserved); CASES_DIR cases are permanently deleted.

Backend:
- DELETE /api/cases/:name — unlink or delete
- PUT /api/cases/order — persist ordering to settings.json
- GET /api/cases now respects saved caseOrder

Frontend:
- Third "Manage" tab in createCaseModal with case list
- Delete button in mobile case picker bottom sheet
- SSE events: case:deleted, case:order-changed

src/web/public/constants.js

@@ -291,6 +291,12 @@ const SSE_EVENTS = {
   ORCHESTRATOR_TASK_FAILED: 'orchestrator:taskFailed',
   ORCHESTRATOR_COMPLETED: 'orchestrator:completed',
   ORCHESTRATOR_ERROR: 'orchestrator:error',
+
+  // Cases
+  CASE_CREATED: 'case:created',
+  CASE_LINKED: 'case:linked',
+  CASE_DELETED: 'case:deleted',
+  CASE_ORDER_CHANGED: 'case:order-changed',
 };
 
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